1. Technical Field
The present invention relates to a capacitance type sensor. More particularly, the present invention relates to a capacitance type sensor such as an acoustic sensor or a pressure sensor which is manufactured using a MEMS (Micro Electro Mechanical Systems) technique.
2. Related Art
(Conventional Acoustic Sensor)
An acoustic sensor is known as a capacitance type sensor which is manufactured using a MEMS technique. FIG. 1A illustrates part of a conventional capacitance type acoustic sensor. This acoustic sensor is provided with diaphragm 13 (movable electrode film) on silicon substrate 11 including penetration hole 12, and has fixed electrode film 16 opposite to diaphragm 13 with a gap provided above diaphragm 13. Fixed electrode film 16 is provided to a bottom surface of dome-shaped protective film 15 provided above penetration hole 12 to cover diaphragm 13. Multiple acoustic holes 17 are opened in protective film 15 and fixed electrode film 16 to introduce acoustic vibration in a gap between fixed electrode film 16 and diaphragm 13.
When acoustic vibration passes through acoustic holes 17 and enters a gap between fixed electrode film 16 and diaphragm 13, the acoustic vibration vibrates diaphragm 13. The vibration of diaphragm 13 changes the capacitance between diaphragm 13 and fixed electrode film 16, and the acoustic vibration is converted into an electrical signal. Further, vent hole 14 is provided between a bottom surface of a periphery portion of diaphragm 13 and a top surface of silicon substrate 11, and a top surface side and a bottom surface side of diaphragm 13 communicate through vent hole 14 to relax a pressure difference between the top and bottom surfaces of diaphragm 13.
An acoustic sensor of such a structure reduces acoustic noise and improves an S/N ratio by reducing an acoustic resistance. It is known that the acoustic resistance of the acoustic sensor becomes high at a narrow portion of a channel for acoustic vibration, and it is known that the acoustic resistance becomes the highest in a vent hole in particular. Consequently, it is effective to reduce an acoustic resistance of a vent hole to reduce an acoustic resistance of an acoustic sensor.
A method of making length L of vent hole 14 short by widening penetration hole 12 of silicon substrate 11 is generally adopted as a method of reducing an acoustic resistance of a vent hole. However, as described below, the method of making length L of vent hole 14 short has reduced drop impact resistance when the acoustic sensor is dropped.
As illustrated in FIG. 1A, even when the acoustic sensor is dropped and diaphragm 13 significantly deflects downward, as long as length L of vent hole 14 is sufficiently secured, a periphery portion of diaphragm 13 (a fixed portion of diaphragm 13 and an intermediate portion of the fixed portion) abuts on the top surface of silicon substrate 11 as indicated by a broken line in FIG. 1A. As a result, a relatively wide range of the periphery portion of diaphragm 13 is supported by the top surface of silicon substrate 11, so that it is possible to suppress deflection of diaphragm 13 and prevent diaphragm 13 from being broken. By contrast with this, when penetration hole 12 is widened as illustrated in FIG. 1B and length L of vent hole 14 is made short, only an end of the periphery portion of diaphragm 13 is supported by the top surface of silicon substrate 11 when the acoustic sensor is dropped as indicated by a broken line in FIG. 1B. As a result, when the acoustic sensor is dropped, diaphragm 13 is likely to significantly deflect downward and be broken.
Thus, reducing an acoustic resistance of an acoustic sensor (improvement of an S/N ratio) and keeping or enhancing a drop impact resistance have a trade-off relationship in a conventional acoustic sensor, and it is difficult to simultaneously enhance both of the acoustic resistance and the drop impact resistance.
(Acoustic Sensor of Patent Document 1)
The acoustic sensor disclosed in Patent Document 1 is provided with air escape portion 19 formed with a vertical hole or a groove whose cross-section is rectangular in the top surface of silicon substrate 11 in vent hole 14 as illustrated in FIG. 2. According to this structure, the cross-sectional area of vent hole 14 partially becomes large at the position of air escape portion 19, so that it is possible to reduce the acoustic resistance of vent hole 14.
However, the acoustic sensor of this structure needs to be manufactured as follows. Air escape portion 19 formed with a vertical hole or a groove whose cross section is rectangular is opened in the top surface of silicon substrate 11 by dry etching, and a lower sacrificial layer, a diaphragm layer and an upper sacrificial layer are stacked on the top surface of silicon substrate 11 to make this acoustic sensor. After these layers are etched to predetermined shapes, protective film 15 and fixed electrode film 16 are formed on the upper sacrificial layer. Next, the bottom surface side of silicon substrate 11 is etched to open penetration hole 12, and the upper sacrificial layer and the lower sacrificial layer are removed by etching to form a gap between diaphragm 13 and protective film 15 and vent hole 14.
The acoustic sensor of Patent Document 1 manufactured in this way needs to make air escape portion 19 deep to some degree to reduce the acoustic resistance of vent hole 14. Therefore, a concavity meeting the position of air escape portion 19 is produced in a top surface of the lower sacrificial layer, and further a concavity is also produced in a top surface of the diaphragm layer. As a result, the shape of air escape portion 19 is transferred to diaphragm 13, and a concave portion and a convex portion are finally produced in the top surface and the bottom surface of diaphragm 13, respectively. These concave portion and convex portion influence a stress state of diaphragm 13 and eventually influence the sensitivity of the acoustic sensor.
However, when the depth of air escape portion 19 is decreased to such a degree that the shape is not transferred to diaphragm 13, it is not possible to provide an effect of reducing the acoustic resistance of vent hole 14.
Further, the acoustic sensor of Patent Document 1 needs to be provided with air escape portion 19 by dry etching as described above, and therefore the number of manufacturing processes increases and manufacturing cost rises.
Patent Document 1: Japanese Unexamined Patent Publication No. 2010-34641 (FIG. 10)